Holograms are recordings in or on photosensitive plates of light intensity patterns created by the interference of two beams of mutually coherent light. Typically, one of the beams is produced by light waves from an object wave and the other beam is produced by light waves from a reference source. The light intensity patterns contain information on both the phase and amplitude of the light waves. This information is in coded form, and the hologram itself bears no resemblance to the object. Instead, the intensity patterns that form the hologram are in essence "fringes" that run through the thickness of the photosensitive plate. After processing, when the hologram is illuminated by the reference beam, light is diffracted from the hologram such that the object beam is reconstructed, thereby generating a wavefront that makes it appear as though the light had originated from the object and thus in essence, creating a three-dimensional image of the object.
There are two major categories of holograms: transmissive and reflective. Transmission holograms are created, in essence, by two wavefronts incident upon the photosensitive plate from the same side. On the other hand, reflection holograms are created by two wavefronts incident upon the photosensitive plate from opposite sides. In transmission holograms, the interference fringes recorded are roughly perpendicular to the photosensitive plate surface, somewhat like the slats of a venetian blind, whereas in reflection holograms, the interference fringes recorded are more nearly parallel with the surface of the plate, like the pages of a book. These two categories are further divided into two physical types of holograms: surface relief holograms and volume phase holograms.
Volume phase holograms work using the same principle as Bragg volume gratings. Bragg volume gratings are made up of multiple layers of material with different refractive indices. In volume phase holograms, the surfaces or "fringes" of these layers which have different refractive indices are created by two plane waves or two waves and are also referred to as "Bragg planes." In essence, volume phase holograms behave as if they consisted of multiple overlapped stacks of Bragg planes.
The direction and wavelength of light that is reflected from a single Bragg grating, and thus a volume phase hologram, depends upon how the layers are tipped and the distance between the layers. An efficient Bragg grating reflects almost all of the light rays that satisfy Bragg's Law, and lets light rays that do not satisfy the law pass through. Bragg's Law states that n.lambda.=2d cos .theta., where n is an integer typically 1, .lambda. is the wavelength of the light ray, d is the distance between the Bragg planes, and .theta. is the angle, known as a "Bragg angle," between the light ray and the Bragg plane normal vector. A range of different wavelengths can satisfy Bragg's Law for a given grating. As determined by the equation, each wavelength of the visible spectrum has a different angle.
A problem with Bragg gratings and thus volume phase holograms, is that Bragg gratings are three-dimensional. Therefore, for a given wavelength .lambda., any Bragg grating can be illuminated within a wide arc of the Bragg angle .theta. centered on the Bragg plane normal. This effect produces an undesirable result when the hologram is illuminated, because light incident on the hologram from other directions, also referred to as ambient light, illuminates the hologram and causes a distortion or a lack of clarity in the desired object image. Therefore, there is a need for a holographic system wherein the acceptable angles of illumination are narrowed.